1. Field of the Invention
The present invention relates to a method and apparatus for forming a diamond film, and more particularly to a chemical vapor deposition (CVD) method of a diamond film by making use of an arc discharge.
2. Description of the Prior Art
Recently, chemical vapor deposition methods for forming a diamond film under low pressure have been noted. Such methods can be roughly classified into the following three types.
The first type is called a heat filament CVD method. According to this method, a tungsten filament heated at 2000.degree. C. or higher is disposed just above a substrate which is at 800.degree. C. through 1000.degree. C., and a mixture gas of hydrogen and hydrocarbon such as methane is sprayed onto the substrate through the heated filament to grow a diamond film on the substrate.
The second type is called a microwave plasma CVD method. According to this method, a plasma is generated in a mixture gas of hydrogen and hydrocarbon by microwave of hundreds of watts to grow a diamond film on a substrate which is positioned within the generated plasma and is heated by the microwave at about 700.degree. C. through 900.degree. C.
In the above described two types of methods, atomic hydrogen accelerates the decomposition of methane, CH.sub.4, and prevents the formation of other synthetic material such as amorphous carbon selectively.
The third type is a method using ion beam. According to this method, a diamond film is growed on the substrate by irradiating the substrate with ion beam of carbon.
The above described conventional forming methods of the diamond film have the problems that the forming speed is only several .mu.m/hr which is considerably slow for forming the diamond film with the thickness of 100 .mu.m or more to be used as a substrate for heat-sink for example.
In the meantime,, the inventors of the present invention found through their research that direct current arc discharge could be effectively used for the fast growth of the diamond film. The arc discharge is characterized in that electron temperature is as high as gas temperature which is more than several thousands.degree.K. Accordingly, the decomposition of a mixture gas of hydrogen and hydrocarbon to be supplied as a plasma source gas is extraordinarily accelerated as compared with the heat filament CVD or microwave plasma CVD using glow discharge. As a result, the growing speed of a diamond film by a method using the direct arc discharge is 10 through 100 times faster than the above-mentioned conventional methods.
As shown in U.S. Pat. No. 4,851,254, the inventors of the present invention proposed a direct current arc method by which a plasma source gas is effectively used. According to this method, a plasma source gas is forced to pass through an arc discharge so as to form a gas plasma, and by aperture it changes to a gas plasma to be sprayed onto a substrate on which the diamond film is to be formed. However, the inventors of the present invention found the following problems about such method through their experiment.
As the plasma jet gas to be sprayed onto the substrate has a high temperature of several thousands.degree.K, the rate of occurrence of impurity carbon (any kinds of carbons other than diamond) such as graphite becomes high when the surface temperature of the diamond film growing on the substrate exceeds 1000.degree. C. under low pressure. Accordingly, both cooling the substrate and controlling the temperature are extremely important regarding the forming method using the plasma jet generated by arc discharge. In reference to this aspect, the inventors of this invention discovered that the surface temperature of the diamond film varies as the thickness thereof increases even if the cooling of the substrate is well done. More specifically, it was found that the surface temperature of the diamond film rises due to the athermalization as the thickness of the film increases (as the forming time increases), irrespective of the character of diamond having a high thermal-conductivity, and that such temperature increase of the film results in the high rate of impurity carbon.
FIG. 1 illustrates the data on the above explanation, indicating the Raman spectrum of the diamond film growth after the lapses of 1 Hr, 5 Hr, and 10 Hr from the start of the growth. Corresponding to the lapse of the forming time, the thicknesses of the diamond film are respectively 30 .mu.m, 150 .mu.m, and 300 .mu.m at each lapse of 1 Hr, 5 Hr, and 10 Hr from the start of the growth. According to the Raman spectrum data in FIG. 1, as the forming time is longer, namely as the diamond film is thicker, a peak of 1600 cm.sup.-1, which indicates the impurity carbon such as graphite and differs from the diamond peak of 1330 cm.sup.-1, is stronger.